U.S. Pat. No. 3,589,167 (Hill), which issued to the assignee of the present invention, discloses a thermomechanical analysis apparatus. The apparatus includes a probe assembly which is suspended in nearly weightless contact with a sample material. Weightlessness is achieved by means of a fluid reservoir and a float which is submerged in the fluid and coupled to the probe. Weights are added manually to the probe assembly to just offset the buoyancy of the float. As the height of a sample under test changes, the nearly weightless probe moves up or down in response.
Vertical movement of the probe assembly of the Hill device is sensed by a conventional linear variable differential transformer (LVDT). The magnetic core of the LVDT is mounted on a non-magnetic rod which supports the probe. In this prior art device, a rack and pinion are provided for moving the LVDT coil assembly to achieve a null prior to beginning a measurement. The rod member which supports the probe passes through a pair of roller bearings. The entire assembly is suspended from a frame at a pair of spaced support points.
U.S. Pat. No. 4,354,764 (Ackermann et al) discloses a similar system in which electromagnetic means is utilized for compensation of the weight of the probe assembly. The electromagnetic means may be adjusted to apply a desired force to the sample, for example for hardness testing. It is further suggested in the Ackermann patent that a force may be applied in a cyclic fashion from which, e.g., the elasticity of the specimen can be determined.
The requirements for the electronic circuitry associated with dynamic oscillatory mechanical analysis apparatus are quite specialized. It is particularly desirable to measure the phase difference between the oscillatory force applied to a test sample (specimen) and the oscillatory dimensional response and, also, to measure changes in the magnitude or amplitude of the response. These determinations are to be effected over a period of time as the test sample is subjected to a varying temperature. Problems associated with the circuitry are related to the very low frequencies of interest, e.g. 0.01 to 100 hertz, and include low signal to noise ratio, drift, and sympathetic oscillations in the sample. There are potentially large and unpredictably variable changes in the DC component due to temperature induced changes in dimension and softness of the sample. There particularly is a need to process the low frequency signals for phase and amplitude independently of frequency, which conventional circuitry does not effect suitably.
Therefore an object of the present invention is to provide electronic circuitry suitable for generating and processing low frequency signals such as for oscillatory mechanical analysis apparatus. A more specific object is to measure phase of a signal relative to a reference signal independently of frequency and independently of a drifting DC component in the signal. A further object is to amplify and filter a low frequency weak signal independently of frequency. Another object is to provide a wave generator for generating a sine-type wave at relatively low cost and with a tunable wave form. Yet another object is to generate an oscillatory wave that has an operating frequency proportional to a control voltage and that may be utilized in a circuit for measuring phase independently of the operating frequency.